Menthol, especially l-menthol, has been an extremely important refreshing flavor and been extensively used. As a synthesis process of menthol, optical resolution of dl-menthol and asymmetric synthesis are known (nonpatent literature 1). In the asymmetric synthesis of l-menthol, l-menthol is produced by hydrogenation of the precursor l-isopulegol. For synthesis of l-isopulegol, the selective cyclization of d-citronellal is an important step.
As the selective cyclization of d-citronellal, techniques which use zinc bromide or other reagents have long been known. In recent years, there are reports on highly selective cyclization using an aluminum complex as a catalyst. For example, highly selective cyclization which uses, as a catalyst, an aluminum complex having a 2,6-diphenylphenol-derived ligand was invented (patent literature 1). Since then, cyclization which uses an aluminum complex having a ligand derived from compounds having a phenolic hydroxyl group (patent literatures 2, 3 and 4), and cyclization which uses an aluminum complex having a silyl ether moiety (patent literature 5) have been reported. However, there is no report on selective cyclization of only one of the two stereoisomers in racemic citronellal using an optically active aluminum complex. Many aluminum complexes having a biaryldiol skeleton, which is an axially asymmetric ligand, have been reported, but among them, only a monovalent cationic hydride complex (patent literature 6) consists of aluminum and biaryldiol at the ratio of 2:3 (aluminum:biaryldiol).